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The Problem With Due Dates

by — Last updated: 2022-02-21 in Pregnancy

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Are you pregnant, or know someone who is? Before you ask when the baby is "due", ask what estimated due dates really mean. A new study just found that pregnancy length naturally varies by as much as five weeks.

We know that babies who are born prematurely are much more likely to have medical problems, and we also know that it is safer to take steps to induce labor than to let a pregnancy continue beyond 42 weeks. But how much does a baby's assigned estimated due date really say about the day on which he or she is likely to be born?

We already knew that very few babies are born right on the due date they were given. If you are going to place a bet on your baby's birth date, your odds will be much better if you do not pick the actual due date. 

Read More: Planning Your Pregnancy
 
Curious why? Well, here's why. Only four percent of babies are born on their due date, and 70 percent are born within 10 days of their due date, meaning either before or after it. 

Pregnancy length appears to vary quite a bit naturally, but how much? Previously, the apparent variations in gestational length were sometimes attributed to faulty due date calculations — in other words, perhaps the mother got the date of her last period wrong. A recent study of 125 women set out to discover natural variations in pregnancy. In this case, the researchers knew the exact dates of conception.

The research team from the US National Institute of Environmental Health Sciences took daily urine samples from women who were trying to conceive naturally in order to find out their ovulation and implantation dates. They were surprised by their own findings:

  • Gestational length naturally varied by five weeks (37 days), even after excluding six premature births.
  • The average gestational length, measured from conception, was 268 days or 38 weeks. 
  • Embryos that took longer from conception to implantation also gestated for longer.
  • Older mothers were more likely to have longer pregnancies. 
  • There was a link between gestational length and maternal weight. 
The researchers' findings left them wondering whether the concept of due dates is useful at all.

What can women who are having babies now take away from this study? Well, to start with, that there is really no need to be induced simply for gestational age once you reach 38 weeks.

The estimated due date really is no more than a somewhat informed estimation. Perhaps pregnant women can learn something from Kate Middleton, who did not offer the public an exact due date. When people ask you when you are "due", you can offer a month-long window instead of an exact date — as in "sometime in June, or maybe early July". You'll be giving a more accurate answer that way, and you will avoid those annoying "is the baby there yet" calls right around your due date.